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Project Comparisons

There are no solutions, only trade-offs.

This page includes both direct pooling alternatives and adjacent storage technologies that solve different problems. Compare their operating layer, failure domain, redundancy model, and platform requirements rather than only their apparent pool capacity.

mhddfs

mhddfs is, like mergerfs, a FUSE-based filesystem which can pool multiple filesystems together.

mhddfs had not been updated in over a decade and has known stability and security issues. mergerfs provides a superset of mhddfs' features and offers better performance. As of 2020, the author of mhddfs has moved to using mergerfs.

  • Better than mergerfs for: Nothing?
  • Worse than mergerfs for: most modern deployments due to maintenance status, feature coverage, and reliability.

Below is an example of mhddfs and mergerfs set up to behave similarly. mhddfs falls back to the drive with the most free space when every drive has less than mlimit free; mergerfs with minfreespace filters those branches instead and may fail the create if none qualify.

mhddfs -o mlimit=4G,allow_other /mnt/hdd/disk0,/mnt/hdd/disk1 /mnt/pool

mergerfs -o minfreespace=4G,category.create=ff /mnt/hdd/disk0:/mnt/hdd/disk1 /mnt/pool

aufs

aufs, another union filesystem, is a kernel-based overlay filesystem with basic file creation placement policies.

Compared with mergerfs, aufs focuses on kernel-level overlay and copy-on-write behavior, while mergerfs focuses on userspace pooling of existing filesystems with richer branch placement policies.

aufs failed to be included in the mainline kernel and is no longer available in most Linux distros, making it harder to get installed for the average user. Development has been largely dormant for years.

While aufs can often offer better peak performance due to being primarily kernel-based (at least when passthrough.io is disabled in mergerfs), mergerfs provides more configurability and is generally easier to use. mergerfs, however, does not offer the overlay / copy-on-write (CoW) features which aufs has.

  • Better than mergerfs for: kernel-level overlay/CoW workloads where that behavior is required.
  • Worse than mergerfs for: users needing a widely packaged, actively-maintained, easy-to-deploy pooling solution.

unionfs

unionfs for Linux is a "stackable unification file system" which functions like many other union filesystems. It was last released for Linux v3.14 back in 2014.

Compared with mergerfs, unionfs is an older kernel union filesystem implementation while mergerfs is a modern, actively-maintained FUSE-based pooling filesystem with broader policy controls.

Documentation is sparse so a comparison of features is not possible but given the lack of maintenance and support for modern kernels there is little reason to consider it as a solution.

  • Better than mergerfs for: historical or legacy environments that already depend on this specific kernel unionfs behavior.
  • Worse than mergerfs for: practical modern deployments due to limited maintenance, aging kernel support, and sparse documentation.

unionfs-fuse

unionfs-fuse is a FUSE-based union filesystem focused on overlay behavior, including copy-on-write (CoW) style layering of writable content over read-only content.

Compared with mergerfs, unionfs-fuse's primary feature is overlay/CoW-style layering, while mergerfs' primary feature is policy-based pooling and placement across multiple regular writable filesystems.

As a FUSE filesystem, unionfs-fuse can often be mounted by an unprivileged user when the system's FUSE configuration permits it. Kernel solutions such as unionfs, aufs, and overlayfs normally require additional privileges.

  • Better than mergerfs for: overlay-style unions with copy-on-write semantics in userspace.
  • Worse than mergerfs for: flexible create-policy-based pooling of many writable filesystems.

overlayfs

overlayfs is effectively the successor to unionfs, unionfs-fuse, and aufs and is widely used by Linux container platforms such as Docker and Podman. Both overlayfs and FUSE were originally created by Miklos Szeredi.

If your use case is layering a writable filesystem on top of read-only filesystems then you should look first to overlayfs. Its feature set however is very different from mergerfs and solves different problems.

  • Better than mergerfs for: container and image-layering workflows where copy-on-write semantics are central.
  • Worse than mergerfs for: heterogeneous pooling use cases that need flexible branch selection policies across many existing filesystems.

fuse-overlayfs

fuse-overlayfs implements overlayfs-style lower, upper, and work directories in userspace. Its primary use case is rootless containers, where a kernel overlayfs mount may not be available.

Like unionfs-fuse and kernel overlayfs, it provides layering and copy-up semantics rather than mergerfs-style placement across a pool of writable filesystems.

  • Better than mergerfs for: rootless container and other unprivileged overlay workloads.
  • Worse than mergerfs for: general storage pooling across independent writable filesystems.

RAID0, JBOD, SPAN, drive concatenation, striping

"JBOD" is ambiguous: it can also mean disks exposed independently with no aggregation. In this section it refers only to concatenated or spanned layouts.

These are block device technologies which, in some form, aggregate devices into what appears to be a single device on which a traditional filesystem can be used. The filesystem has no understanding of the underlying block layout and, should one of those underlying devices fail or be removed, the filesystem will be missing that chunk, which could contain critical information, and the whole filesystem may become unrecoverable. Even if the data from the filesystem is recoverable, doing so may require specialized tooling.

In contrast, with mergerfs you can format devices as you normally would, or take existing filesystems and combine them in a pool to aggregate their storage. The failure of any one device will have no impact on the other devices. The downside to mergerfs' approach is that you do not actually have contiguous space as large as you would with those other technologies. That means you cannot create a file larger than 1TB on a pool of two 1TB filesystems.

  • Better than mergerfs for: creating one contiguous block device and maximizing single-file size/performance characteristics.
  • Worse than mergerfs for: failure isolation and straightforward per-disk recovery when a device dies.

RAID5, RAID6

RAID5, RAID6, and similar technologies aggregate multiple devices at the block level and provide one or more live parity schemes that can allow the pool to continue to run should one device fail and rebuild the data once the device is replaced.

mergerfs offers no parity or redundancy features so in that regard the technologies are not comparable. SnapRAID or NonRAID (see below) can be used in combination with mergerfs to provide redundancy. Unlike traditional RAID5 or RAID6, SnapRAID works with drives of different sizes and can have more than 2 parity drives. Parity calculations with SnapRAID are not done in real-time but NonRAID's are.

For more details and comparison of SnapRAID to related technologies see https://www.snapraid.it/compare.

  • Better than mergerfs for: live parity protection and continued array operation through configured disk failures.
  • Worse than mergerfs for: incremental mixed-size pooling with simple per-filesystem independence.

LVM

LVM is a block-level volume management system for Linux. It combines physical devices into volume groups and creates logical volumes on top, with features like snapshots, resizing, and thin provisioning.

Compared with mergerfs, LVM operates below the filesystem while mergerfs operates above filesystems. With LVM, filesystems are created on logical volumes and do not know about the underlying disks. If a linear or striped logical volume spans multiple disks and one disk fails, the filesystem can become unreadable. With mergerfs, each disk typically has its own independent filesystem, so a single disk failure affects only the data on that disk.

  • Better than mergerfs for: block-level features like thin provisioning, snapshots, and online resizing.
  • Worse than mergerfs for: incremental mixed-disk pooling where per-disk failure isolation and per-disk native access are priorities.

mdadm

mdadm is Linux userspace tooling for configuring and managing software RAID arrays provided by the kernel md driver. It can build RAID0/1/4/5/6/10 and related layouts, monitor arrays, and handle rebuild workflows.

Like RAID technologies discussed above, mdadm arrays aggregate devices at the block layer and present one virtual device to a filesystem. That can provide performance and/or redundancy depending on RAID level, but it also means the filesystem depends on the array layout. mergerfs does not provide block-level RAID; it pools already-formatted filesystems and can be combined with tools like SnapRAID or NonRAID when redundancy is needed.

  • Better than mergerfs for: kernel software RAID with real-time redundancy and array-level performance tuning.
  • Worse than mergerfs for: flexible mixed-size drive pooling where per-filesystem independence and simple incremental expansion matter.

device mapper

Device mapper is a kernel framework for creating virtual block devices from one or more underlying devices. Technologies such as LVM, dm-crypt, dm-cache, dm-thin, and multipath are built on top of it.

Because device mapper works at the block layer, it is not directly comparable to mergerfs. It is complementary: you can use device mapper for block-level functionality (encryption, caching, thin provisioning, etc.) and then place filesystems on those devices, which mergerfs can later pool if desired.

  • Better than mergerfs for: block-layer transformations such as transparent encryption, caching, and provisioning features.
  • Worse than mergerfs for: presenting one unified pathname-level view across many independent, already-formatted filesystems.

Unraid

Unraid is a full OS platform that offers pooled storage with integrated real-time parity. Compared with mergerfs, its main advantage is that it bundles pooling and parity in one product, while mergerfs is a standalone Linux pooling filesystem without built-in redundancy. For some users mergerfs' Linux distro-agnostic and fully open-source approach is also preferable.

For semi-static data, mergerfs + SnapRAID provides a similar, but not real-time, solution. NonRAID (see below) is a fork of Unraid's parity calculation solution and can also be used with mergerfs.

Moving between the two is not symmetric because mergerfs membership is a configuration change while Unraid array membership is part of the array state. See Migration for details.

  • Better than mergerfs for: turnkey pooling plus integrated real-time parity in one product.
  • Worse than mergerfs for: non-destructive incremental addition and removal of populated filesystems, arbitrary branch types and filesystem layouts, and users who prefer a Linux distro-agnostic, fully open-source component approach.

NonRAID

NonRAID is a fork of Unraid's storage array kernel driver. Effectively it is like RAID5 at the block device layer without treating the collection of devices as a single device. Each device gets a virtual block device associated with it which can be formatted with the filesystem of the user's choice and will be individually accessible. Similar to SnapRAID but real-time.

Compared with mergerfs, NonRAID adds kernel-level real-time parity while still allowing per-disk filesystems, whereas mergerfs focuses on pooling views and placement policies without parity.

Given each device has its own filesystem like a traditional setup it can be used with mergerfs for those looking for a unified view.

  • Better than mergerfs for: real-time parity protection without traditional RAID striping behavior.
  • Worse than mergerfs for: users who want the simplest setup and do not need parity, since it adds operational complexity.

ZFS

ZFS is an advanced filesystem that combines storage pooling, snapshots, data integrity checking, and built-in RAID-like redundancy all in one package. It offers features like transparent compression, encryption, and automatic data repair.

Compared with mergerfs, ZFS is a full storage stack that manages disks directly with integrated integrity and redundancy features, while mergerfs is a lightweight FUSE layer that pools existing filesystems without built-in redundancy.

  • Better than mergerfs for: integrated data integrity, snapshots, checksumming, and built-in redundancy.
  • Worse than mergerfs for: low-overhead incremental pooling of existing filesystems with minimal planning.

ZFS AnyRAID

ZFS AnyRAID is under development to support mixed-capacity disks while retaining ZFS redundancy. The planned layouts include mirrored and RAID-Z variants, along with rebalancing and pool contraction work.

Availability is still landing in OpenZFS. Published notes indicate initial mirror layouts may ship in a 2.4.x release, with broader RAID-Z variants, rebalancing, and pool contraction targeted for later releases. Treat release and downstream product-integration dates as roadmap targets rather than commitments.

Where available, AnyRAID would offer some of the mixed-size flexibility associated with mergerfs while retaining redundancy inside ZFS. mergerfs provides mixed-size, add-drives-anytime pooling today, but without built-in redundancy; SnapRAID or NonRAID can provide separate protection.

  • Better than mergerfs for: users who want mixed-capacity flexibility and built-in live redundancy in one filesystem stack.
  • Worse than mergerfs for: users who need production availability now or want to pool existing filesystems without migrating data.

bcachefs

bcachefs is a modern copy-on-write filesystem for Linux designed for multi-device storage aggregation. It includes integrated features such as checksumming, snapshots, compression, encryption, and redundancy.

Compared with mergerfs, bcachefs is a full integrated filesystem, while mergerfs is a lightweight pooling layer over existing filesystems. Beginning with Linux 6.18, bcachefs is no longer distributed in the mainline kernel; the project now distributes it as an external DKMS module. It remains actively developed, but deployments must account for kernel compatibility and its shorter operational history.

  • Better than mergerfs for: integrated CoW filesystem features, checksumming, snapshots, compression, and built-in multi-device redundancy.
  • Worse than mergerfs for: pooling existing filesystems without reformatting, mainline-kernel availability, and per-disk native access.

Btrfs multi-device filesystems

Btrfs can combine multiple devices in one filesystem and select separate allocation profiles for data and metadata. The single data profile spreads one copy of data across available devices without redundancy; other profiles provide mirroring, striping, or parity with different capacity and failure trade-offs.

Compared with mergerfs, multi-device Btrfs is one integrated filesystem, while mergerfs pools separate existing filesystems. Losing a device from a single profile can make the whole Btrfs filesystem incomplete. Redundant profiles change that risk, but the member devices still are not independently mountable filesystems.

  • Better than mergerfs for: integrated checksumming, snapshots, balancing, and selectable redundancy profiles.
  • Worse than mergerfs for: pooling existing mixed filesystem types, per-disk native access, and failure isolation without redundancy.

StableBit's DrivePool

DrivePool is a commercial storage pooling product for Windows that combines multiple drives into a single volume and includes balancing and optional duplication capabilities.

Compared with mergerfs, DrivePool emphasizes integrated balancing and duplication features in a Windows-first product, while mergerfs emphasizes Linux FUSE pooling with policy-based create placement and external-tool composability.

  • Better than mergerfs for: Windows users needing built-in rebalancing and native file duplication features.
  • Worse than mergerfs for: Linux-native deployments and users who prefer open-source tooling and composable external utilities.

Windows Storage Spaces

Windows Storage Spaces is Microsoft's block-level storage virtualization platform for Windows. It aggregates physical disks into storage pools from which virtual disks can be created with simple, mirror, or parity layouts. Supported Windows Server configurations can also use faster media as a Storage Bus Cache.

Unlike mergerfs, Storage Spaces manages the member devices below the filesystem and stripes or mirrors data according to the selected layout. This can provide integrated resilience and one contiguous volume, but member disks are not independently readable filesystems. mergerfs instead pools existing filesystems and leaves their files directly accessible.

  • Better than mergerfs for: Windows environments needing integrated pooling, resilience, and tiering in the platform stack.
  • Worse than mergerfs for: Linux hosts and lightweight pooling where minimal overhead and per-filesystem independence are preferred.

Plan 9 binds

Plan 9 has a native bind mechanism that can compose paths and filesystems into a single namespace view, including union-like directory behavior.

Compared with mergerfs, Plan 9 binds are an OS-native namespace feature with simple ordered behavior, while mergerfs provides Linux-focused filesystem pooling with a larger set of configurable branch policies.

Plan 9 isn't a widely used OS so this comparison is mostly academic.

  • Better than mergerfs for: native Plan 9 environments where namespace composition is a core OS feature.
  • Worse than mergerfs for: Linux users due to ecosystem relevance and practical deployment availability.

9P

9P is a filesystem protocol from the Plan 9 operating system used to access files over a network between systems.

Compared with mergerfs, 9P is a network filesystem protocol, while mergerfs is a local Linux FUSE filesystem for pooling directories and filesystems under one mount with branch policies. Like the Plan 9 binds comparison above, this is mostly academic for typical mergerfs users.

  • Better than mergerfs for: lightweight network filesystem protocol use between compatible systems.
  • Worse than mergerfs for: local Linux filesystem pooling because it does not provide mergerfs-like branch policies or pooling semantics.

SnapRAID pooling

SnapRAID has a pooling feature which creates "a read-only virtual view of all the files in your array using symbolic links."

Compared with mergerfs, SnapRAID pooling is a symlink-generated, read-only view refreshed by SnapRAID operations, while mergerfs is a live mounted filesystem that supports normal read/write behavior.

As mentioned in the description, this "view" is just the creation of the same directory layout with symlinks to all files. This means that reads (and writes) to files are at native speeds, but it is not practical to use as a target for writing new files, and it is only updated when snapraid pool is run. Note that some software treats symlinks differently than regular files. For instance, some backup software will skip symlinks by default.

mergerfs has the feature symlinkify which provides a similar behavior but is more flexible in that it is not read-only. That said there can still be some software that won't like that kind of setup.

  • Better than mergerfs for: read-only, native-speed directory views of SnapRAID-managed content.
  • Worse than mergerfs for: general read/write pooled filesystem use and always-live view updates.

rclone union

rclone's union backend allows you to create a union of multiple rclone backends and was inspired by mergerfs. Because rclone knows more about the underlying backends than mergerfs can, it can be more efficient than creating a similar union with rclone mount and mergerfs.

Compared with mergerfs, rclone union is backend-aware and optimized for cloud/remote storage abstractions, while mergerfs is optimized for local POSIX filesystem pooling and branch policies.

However, it is not uncommon to see users set up rclone mounts and combine them with local or other remote filesystems using mergerfs given the differing feature sets and focuses of the two projects.

  • Better than mergerfs for: remote-backend-heavy workloads where backend-aware behavior improves cloud storage handling.
  • Worse than mergerfs for: local POSIX pooling scenarios where direct filesystem semantics and broad local tool compatibility matter.

distributed filesystems

Distributed remote filesystems come in many forms. Some offer POSIX filesystem compliance and some do not. Some provide remote block devices or object stores on which a POSIX or POSIX-like filesystem is built. Some are effectively distributed union filesystems with duplication.

Compared with mergerfs, distributed filesystems focus on multi-node storage, replication, and cluster behavior, while mergerfs focuses on single-host local pooling of existing filesystems.

  • Better than mergerfs for: multi-node or geographically distributed workloads with built-in replication and high availability.
  • Worse than mergerfs for: single-host and homelab pooling where low overhead and operational simplicity are key.

nofs

nofs is a tool that provides mergerfs-like functionality for combining multiple filesystems/directories into a unified view but does so entirely through subcommands which replicate traditional POSIX commands rather than providing a proper filesystem. It takes a lot of inspiration from mergerfs in that it supports policy-based branch selection (mfs, ff, pfrd, epmfs, and others), read-only and no-create branch modes, and recreates POSIX-like commands such as ls, find, which, cp, mv, and rm.

Compared with mergerfs, nofs is command-driven rather than a mounted filesystem, so mergerfs is the option for transparent system-wide pooling through normal POSIX filesystem calls.

Given its design nofs is not suited for general usage as third-party applications will not be able to take advantage of the unioning behavior it offers. It is primarily for simpler situations where mergerfs cannot be used.

  • Better than mergerfs for: constrained environments where a FUSE mount is not possible but command-style union behavior is acceptable.
  • Worse than mergerfs for: transparent system-wide filesystem pooling usable by arbitrary third-party applications.

PolicyFS

PolicyFS is a Linux FUSE storage daemon that unifies multiple storage paths under a single mountpoint, similarly to mergerfs. Its distinguishing features are explicit path-pattern-based routing rules for placement, and an optional SQLite metadata index that can serve readdir and getattr operations without spinning up HDDs.

Compared with mergerfs, PolicyFS emphasizes explicit path-based routing, scheduled SSD-to-HDD movement, and optional metadata indexing. mergerfs emphasizes a broader policy-driven pooling model with longer operational history and wider application compatibility. mergerfs does not try to prevent metadata operations from waking branches because doing so fundamentally conflicts with other features.

PolicyFS's indexed storage deliberately defers some delete, rename, and attribute operations until a maintenance job runs. Out-of-band changes require the index to be refreshed, and its documentation identifies limits around O_DIRECT, advisory locks, NFS re-export, and full POSIX coverage. Those are material trade-offs, not only maturity differences.

  • Better than mergerfs for: explicit path-based routing, scheduled tiering, and metadata indexing designed to reduce HDD spin-ups.
  • Worse than mergerfs for: broad POSIX compatibility, immediate mutation semantics on every branch, and long-established operational behavior.

Greyhole

Greyhole is an open-source storage pooling application that works via Samba. Rather than implementing a FUSE filesystem, it hooks into Samba VFS to intercept file operations and distribute files across multiple drives, with optional per-share redundancy.

Compared with mergerfs, Greyhole is Samba-centric and includes integrated share-level redundancy, while mergerfs is a general-purpose Linux FUSE mount for local pooling via standard POSIX filesystem calls.

  • Better than mergerfs for: Samba-centric environments that need built-in per-share redundancy in the same tool.
  • Worse than mergerfs for: general-purpose local filesystem pooling outside Samba, and for heavy small-file churn workloads.